In scanning electron microscopy, a beam of electrons is scanned over a specimen, and the resulting electrons that are returned from the specimen surface are used to create an image of the specimen surface. In some systems, the beam is arbitrarily controllable to make multiple scan passes over specific areas or portions of areas at different sample frequencies to magnify the image of the surface.
On a specimen made of a substantially insulative material (e.g., a semiconductor material), performing multiple scans over the same small area may cause the specimen to accumulate an excess positive charge in that small area relative to the rest of the scanned area. That excess charge causes an image of that small area to appear dark, thus obscuring image features in that small area.